Generally, handover means that communication which is being performed is maintained as it is when a mobile station goes out of a base station and enters another base station. In other words, handover means that exchange of channel or line is performed to maintain call which is being performed.
Handover can be divided into softer handover, soft handover, and hard handover.
In more detail, softer handover is handover performed within one cell, and means that a mobile station converts a current channel of channels used within cell coverage to an excellent channel. Soft handover means that, if a mobile station intends to convert one channel to another channel, the mobile station tries connection to another channel in a state that connection with one channel is maintained, and finally disconnects connection with one channel. Hard handover means that a mobile station disconnects a current channel in which communication is being performed and directly connects another channel.
The mobile station performs a ranging procedure after performing the aforementioned handover procedure.
Ranging means a set of processes for maintaining connection quality (particularly, synchronization) of RF communication between a base station and a mobile station in IEEE 802.16 or mobile Internet. Particularly, during OFDMA/TDD based multiple access, since a base station transmits data to several mobile stations based on one reference timing in a downlink, there is no problem in the downlink. However, since several mobile stations respectively transmit a signal in an uplink, radio wave delay may occur differently for each of the mobile stations. In this respect, as a separate timing synchronization method, a ranging procedure is performed between the base station and the mobile station. Namely, ranging is a procedure for exactly controlling synchronization of transmission time in several mobile stations.
Hereinafter, a ranging procedure will be described with reference to FIG. 1a to FIG. 2b. 
In case of a general ranging procedure illustrated in FIG. 1a, a mobile station (MS) transmits a desired random ranging code to a target base station (TBS) (S13), and receives a ranging response message from the target base station if the transmitted ranging code is effective (S14).
However, as illustrated in FIG. 1b, if a plurality of mobile stations request one target base station to allocate the same ranging code, collision of the ranging codes may occur among the plurality of mobile stations (S13). In this case, since the ranging procedure should be retried after the lapse of a certain time period, handover interruption time may increase rapidly.
In addition, in case of a fast ranging procedure illustrated in FIG. 2a, the mobile station (MS) is allocated with an uplink resource from the target base station (TBS) through Fast_Ranging_IE (Information Element) included in an uplink map message (S22). Accordingly, the mobile station can transmit a ranging request RNG_REQ message to the target base station (S23) and receive a ranging response RNG_RSP message from the target base station (S24).
However, as illustrated in FIG. 2b, the target base station may fail to receive the ranging request message RNG_REQ (S23), or the mobile station may fail to receive the ranging response message RNG_RSP. In this case, since the ranging procedure should retried after the lapse of a certain time period and also the general ranging procedure not the fast ranging procedure should be retried, handover interruption time increases.
Accordingly, in order to complete the ranging procedure within the maximum handover interruption time which is previously designated, it is necessary to reduce the handover interruption time caused by either collision of the ranging codes among the plurality of mobile stations or failure in receiving the RNG_REQ/RNG_RNP messages.